Friction welding machines, both of the power-driven type and of the inertia type, are known in the art. In either type, a pair of parts to be welded are subjected to high-speed relative rotation and are then forced together axially until the heat of friction fluidizes their contact surfaces and welds them together.
Because of the fluidity of the contact surfaces, the pressure with which the parts are forced into contact with one another (usually called the upset pressure) causes the formation of a flash at the weld, and a consequent shortening (usually called the upset) of the welded piece with respect to the combined length of its parts.
The amount of upset in any given welding operation depends on three factors: (1) the upset pressure; (2) the length of time a frictional contact persists; and (3) the speed of relative rotation between the two parts. The upset pressure is a complex process control parameter and is not a suitable variable for the purposes contemplated herein. In friction welders of the inertial type, in which the weld is made by converting all the kinetic energy of a freely spinning flywheel into frictional heat and thereby stopping the flywheel, the time is not independently controllable. Consequently, the most practical parameter to control is the weld speed, i.e., in the case of an inertial welder, the initial rotational velocity of the flywheel when upset pressure is first applied.
U.S. Pat. No. 3,714,509 to Coleman et al describes apparatus for controlling, among other things, the weld speed of an inertia welder as a function of a manually selectable reference voltage. By varying this reference voltage, the amount of upset produced by the weld can be accurately adjusted. However, unless the speed control is manually readjusted before each weld, or unless the lengths of the parts to be welded are held within uneconomically strict tolerances, the finished pieces produced by the apparatus of the Coleman patent will vary in overall length through a range equal to the sum of the tolerances of the individual parts to be welded together. No practical solution to this problem in automated production equipment has heretofore been known.